While meat can contain both natural and artificial hormones, which are often added to produce faster-growing and more productive animals, researchers are now studying how plant-based foods can affect hormone levels. Researchers at the University of California, Berkeley, are examining how certain plant-based foods that are high in estrogenic compounds can influence the behaviors of various primates and how that may have contributed to the evolution of the organisms.

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The research is the first to observe the connection between plant-based phytoestrogens, and behavior in a group of wild red colobus monkeys.

The study found that the more male monkeys dined on the leaves of Millettia dura, a tropical tree containing estrogen-like compounds, the higher their levels of estradiol and cortisol. Because of these altered hormone levels, more acts of aggression and sex occurred, and less time was spent grooming.

The study, published in the current issue of the journal Hormones and Behavior, suggests an important relationship between consuming phytoestrogens and primate evolution.

"It's one of the first studies done in a natural setting providing evidence that plant chemicals can directly affect a wild primate's physiology and behavior by acting on the endocrine system," said study lead author Michael Wasserman, who conducted the research as a graduate student at UC Berkeley's Department of Environmental Science, Policy and Management. "By altering hormone levels and social behaviors important to reproduction and health, plants may have played a large role in the evolution of primate — including human — biology in ways that have been underappreciated."

Researchers followed a group of red colobus monkeys in Uganda and recorded what the primates ate while counting the number of chases and fights, the frequency of mating and time spent grooming. To assess changes in hormone levels, the researchers collected fecal samples to analyze estradiol and cortisol levels.

The Millettia dura is a close relative of soy and "with all of the concern today about phytoestrogen intake by humans through soy products, it is very useful to find out more about the exposure to such compounds in living primates and, by analogy, human ancestors," said study co-author Katharine Milton, professor in UC Berkeley's Department of Environmental Science, Policy and Management. "This is particularly true when determining the influence of phytoestrogens on reproductive behavior, which is the whole keystone of natural selection."